Stable isotope-based hydrological process understanding of a spring catchment in the lesser Indian Himalayas.

Abstract

Springs are hydrologically and ecologically important for the communities in the Indian Himalayan region (IHR). However, spring flows have declined in the past three decades due to the impact of human pressures and climate change, threatening the region's water security. Quantifying and understanding spring aquifer dynamics in complex hydrogeological systems is challenging. And limits our ability to implement frameworks to preserve ecosystem functions. In this study, we characterize the recharge sources and flow processes in a sandstone mountain spring catchment - Paligaad (59.7 km2) in Uttarakhand, India, using stable isotopes of water (δ18O and δ2H). Water samples collected from rainfall, springs, and streams between December 2021 and August 2023 were analyzed for their isotopic composition. Rainfall input was characterized using samples collected at three stations at varying altitudes (amount-weighted monthly samples). A local meteoric water line (LMWL) was characterized as δ2H = 7.8 * δ18O + 11.6 (R2 = .99) for the observation period. The isotopic composition of rain shows seasonality, suggesting that different sources of water vapor cause rains in monsoon and in the dry season. Samples were collected bi-monthly from ten springs (S02, S06, S07, S08, S09, S13, S14, S15, S16, S17), and rainfall responses of individual springs were studied to identify connections to unconfined and deep groundwater strata. Also, S06 was instrumented for monitoring sub-daily discharges. Mean isotope values of the sampled springs ranged from −8.61‰ to −9.68‰ and from −54.93‰ to −61.77‰, for δ18O and δ2H, respectively. The isotopic signature of the spring sources falls closely above the LMWL, indicating a strong rainfall contribution to springs. The deuterium excess values between 15.5‰ and 12.8‰ for the rain samples correspond to far transport of water-bearing clouds. Rainfall isotope composition at the three stations indicates an altitudinal lapse rate of 0.52‰/100m for δ18O. It suggests a probable maximum recharge elevation of 2545, 2436, 2189, and 2216 m.a.s.l for springs S02, S06, S07, and S08, respectively, which lie close to the surface-water catchment ridgeline. The stable isotope data show that input variability (space-time and Hydro-meteorology) impacts the recharge altitude estimates of the spring catchment. The time series of springs showed seasonal patterns and short-term influences, reflecting the contribution from increased recharge activity and tortuous subsurface flow components during monsoons.  In this work, stable isotopes uniquely characterized the springs located close together and estimated recharge catchment altitudes, allowing for water security and risk management. The government agencies will employ the evidence-based process information and the demarcated recharge zones to implement interventions and improve the resiliency and reliability of springs in the IHR. Such experimental research bolsters the understanding of mountain hydrological processes and supports catchment management decision-making across the Indian Himalayas.